The current trend of radiation therapy is moving toward precise dose delivery, which employs an extensive degree of radiation beam intensity modulation and on-line and real-time imaging. The IMRT involves various sources of uncertainty associated with multi-leaf collimator (MLC) positional reproducibility, beam output stability, mechanical isocentricity, and insufficient beam modeling in treatment planning. As a result, the dose delivered to a patient does not accurately represent the dose distribution generated from treatment planning. Therefore, verifying the delivery of a planned radiation dose is required for successful radiation therapy. It is done mostly by comparing the dose distribution from the planning with that from the measurement of the dose delivery. The verification idealistically should be accurate, representative of actual treatment (measurement condition), demand little labor, and require short processing time. Although diverse methods of the verification exist, they can be classified whether the measurement is done before or during treatment. In the following, a brief review is provided.
A group of methods exist, which are based on measurement prior to treatment. The measurement mostly is done on a dosimeter such as x-ray film or an EPID (electronic portal image device) embedded in phantom slabs, and thus are provided with a scattering condition. As such a condition simulates in-patient environment, this group offers true dosimetric verification.
Another group of methods exist that utilizes measurement during treatment. The measurement is to detect transmitted radiation through a patient under treatment. The treatment can be verified (forward verification) in the EPID by comparing the dose image from calculation with that from measurement. Furthermore, the dose can be reconstructed (inverse verification) in a patient from dose image in the EPID, using a computational technique on patient anatomy provided by computer tomography, or CT. Therefore, this group is more representative of actual treatment than the previous group, provided that the real anatomical information of a patient under treatment can be acquired. In addition, this group contributes to adaptive radiation therapy by providing the reconstructed dose. While this group does not achieve true dosimetric verification by employing measurements in a scatter environment, it does fulfill the verification of the dose delivery. This group eliminates human efforts for measurement setup prior to treatment, which is contrary to that required for the first group. As verification involves the two components of calculation and measurement, any methods thus in their application are associated with uncertainty and inaccuracy typically involved in measurements and calculations. When it comes to calculations, the calculation methods for both groups are mostly based on moderately accurate algorithms: calculation algorithms are less accurate than the Monte Carlo (MC) radiation transport method. This in particular challenges the second group in modeling scatter behind a patient. To account for the scatter, an iterative approach, between the two calculation planes of a patient and the EPID, has been made for the dose reconstruction. For this method, convergence is not guaranteed in theory, although good agreements were shown as compared with measurements. On the contrary, in the other forward approach, the scatter could be accurately accounted for in the form of kernels calculated by the MC technique for various EPIDs.